What is Intensity Modulated Radiation Therapy (IMRT)

Intensity modulated radiation therapy (IMRT), is a state-of-the-art technique that uses the computer-controlled multi-leaf collimator to sculpt the radiation treatment beam to the unique tumor shape in order to deliver optimal doses of radiation directly to the tumor. Our targeted methods are much more precise than with conventional radiotherapy.

The level of normal tissue sparing achieved with IMRT is dramatic. While delivering the dose directly to the tumor, we can avoid important normal structures such as the eyes, brain, spinal cord, kidneys, lungs, or heart when the tumor is closely associated with any of these organs. Lower doses of radiation to normal tissues means a tremendous reduction in side effects.

IMRT incorporates several beams coming from different angles to envelope the tumor and the dose intensity of each beam is modulated by changing the shape of the fields during treatment. Advanced imaging services using computed tomography (CT) and magnetic resonance imaging (MRI) are used to help deliver the radiation accurately and precisely. These diagnostic images of the patient’s tumor and surrounding anatomy are uploaded to an advanced treatment planning computer program (images below). This program then generates a treatment plan based on the individual patient’s tumor size, shape, and location within the body, along with the veterinary radiation oncologist’s dose prescription.

Image depicting an IMRT plan with treatment fields from different angles and the dose distribution to the areas as indicated by the different colors. Note that the blue areas in the image have the least amount of dose exposure and the red area receives at least 95% of the dose (the red area is that of the primary tumor).

Our linear accelerator is equipped with a computer-controlled beam-shaping device called a multi-leaf collimator (MLC). The MLC has 120 tungsten metal leaves for sculpting the radiation beam to deliver treatment doses to even the smallest of areas in accordance with the treatment plan. With these developments we are able to treat types of cancer that were once considered untreatable with radiation therapy.

The linear accelerator has the ability to rotate around the patient allowing us treat from multiple angles in order to administer high (optimal) doses of radiation to the tumor while sparing important healthy tissues. A computer program controls the linear accelerator and the MLC (multi-leaf collimator) to match the patient’s treatment plan. Accessories may be added to the treatment plan to improve dose distribution including wedges, blocks and tissue equivalent material (bolus). Positional devices are also used to ensure accurate positioning of the patient, therefore ensuring the dose is delivered to the tumor, for consecutive treatments.

3-D Treatment Plan: dose distribution is equal across all fields. Notice from this image that there is higher exposure of radiation to the normal tissues as compared to the image to the right. For example, the eyes receive more exposure in this 3-D plan. Remember the red, orange and yellow areas receive 90% to 110% of the dose.

IMRT Plan: incorporates several beams from different angles with varying dose intensity per beam. Notice the normal tissue sparing as compared to the image to the left. There are more areas of green and blue (blue indicates 0-40% dose exposure). There is a sharp fall off in dose over the eyes (outlined in dark blue) and lower jaw as compared to the treatment plan on the left.